1. Field of the Invention
The present invention relates to a glass substrate for a display which is particularly useful as a substrate for a flat panel display such as a plasma display panel (PDP) or a field emission display (FED).
2. Discussion of Background
In recent years, a flat panel display, particularly PDP which is one type of thin flat plate type gas discharge display panels, has attracted an attention and has been actively developed. PDP has a cell constituted and defined by a front glass substrate, a rear glass substrate and partition walls, so that plasma discharge is generated in the cell, whereby a phosphor layer on an inner wall of the cell will emit light to form an image.
For the front glass substrate and the rear glass substrate for PDP, a glass having a strain point higher than the strain point of soda lime silica glass, has been employed in order to minimize deformation of the glass substrate during the heat treatment step in the process for the production of PDP.
However, the conventional glass substrate having a high strain point has had a problem that it is susceptible to fracture in the process for the production, as compared with the soda lime silica glass substrate.
It is an object of the present invention to provide a glass substrate for a display, whereby this problem can be solved.
The present invention provides a glass substrate for display, which has a strain point of at least 550xc2x0 C., an average linear expansion coefficient of from 65xc3x9710xe2x88x927 to 100xc3x9710xe2x88x927/xc2x0C. within a range of from 50 to 350xc2x0 C. and a ratio KIC/d of at least 280 Paxc2x7kgxc2x7m7/2 where KIC is the fracture toughness and d is the density.
The present inventors have analyzed in detail the substrates fractured during the process for the production of PDP. As a result, with most of the fractured substrates, impact impressions have been observed which are considered to have formed when the edge portions have received a shock during the handling in an upstream step in the process for the production of PDP. It has been found that with such impact impressions serving as fracture origins, the substrates have been fractured by e.g. a thermal stress in the process for the production of PDP. Accordingly, it is considered possible to solve the problem of fracture of the substrate in the process for the production of PDP by employing a substrate whereby such impact impressions will scarcely form.
The present inventors have investigated the relation between the probability for formation of the above-mentioned impact impressions and the physical properties of the glass substrate, and as a result, have found that the probability for formation of the impact impressions depends on the ratio KIC/d where the KIC is the fracture toughness of the glass and d is the density. Namely, such a probability decreases as KIC/d increases.
The reason is considered to be explained as follows.
KIC is a quantity representing the resistance against fracture of the glass and shows the degree of resistance against the same impact force.
On the other hand, the impact force exerted to a substrate during the handling of the substrate, is represented by the impulse given to the substrate i.e. the change of momentum. The momentum is given by mass xc3x97velocity. Accordingly, the impulse exerted to a substrate having a certain velocity change created, is determined by its mass. For example, the impact force given to a substrate of the same size dropped from the same height, is greater as the specific gravity of the substrate is larger.
On the basis of the foregoing discovery, the present inventors have arrived at the present invention.
The strain point of the glass substrate for a display of the present invention (hereinafter referred to imply as the glass substrate of the present invention) is at least 550xc2x0 C. If it is less than 550xc2x0 C., deformation during the heat treatment step tends to be large. It is preferably at least 560xc2x0 C.
The average linear expansion coefficient of the glass substrate of the present invention within a range of from 50 to 350xc2x0 C. is from 65xc3x9710xe2x88x927 to 100xc3x9710xe2x88x927/xc2x0C. If it is less than 65xc3x9710xe2x88x927/xc2x0C. or more than 100xc3x9710xe2x88x927/xc2x0C., matching in the expansion coefficient with the material which is commonly used for the production of a display (such as glass frit) tends to be difficult. It is preferably from 75xc3x9710xe2x88x927 to 95xc3x9710xe2x88x927/xc2x0C., more preferably from 80xc3x9710xe2x88x927 to 90xc3x9710xe2x88x927/xc2x0C. Hereinafter, the average linear expansion coefficient within a range of from 50 to 350xc2x0 C. will be referred to simply as an expansion coefficient.
In the glass substrate of the present invention, the ratio KIC/d where KIC is the fracture toughness and d is the density, is at least 280Paxc2x7kgxc2x7m7/2. If it is less than 280 Paxc2x7kgxc2x7m7/2, the glass substrate tends to be susceptible to fracture. It is preferably at least 290 Paxc2x7kgxc2x7m7/2.
The glass substrate of the present invention is preferably one prepared by a float method which is capable of producing plate glass of high quality in a large amount.
The glass substrate of the present invention preferably consists essentially of:
More preferably, the glass substrate of the present invention consists essentially of:
The reasons for defining the above compositions will be described below, wherein wt % will be represented simply as %.
SiO2 is essential as a network former. If it is less than 45%, the heat resistance or the chemical durability tends to deteriorate. It is preferably at least 51%. If it exceeds 70%, KIC/d tends to be too small. It is preferably at most 65%, more preferably at most 60%, particularly preferably at most 55%.
Al2O3 is essential as a component to increase the strain point. If it is less than 2%, the strain point tends to be too low. It is preferably at least 6%, more preferably at least 10%. If it exceeds 20%, the viscosity of the molten glass tends to be too high, whereby forming, particularly float forming, tends to be difficult. It is preferably at most 16%.
B2O3 is not essential, but may be incorporated up to 6% to increase KIC/d or to lower the viscosity of the molten glass at the time of melting the glass. If it exceeds 6%, the strain point tends to be too low. It is more preferably at most 5%. When B2O3 is incorporated, the content is preferably at least 0.5%, more preferably at least 1%, particularly preferably at least 2%.
MgO is essential as a component to increase KIC/d and to lower the viscosity of the molten glass at the time of melting the glass. If it is less than 1%, KIC/d tends to be too small, or the viscosity of the molten glass at the time of melting tends to be too large. It is preferably at least 2%, more preferably at least 3%. If it exceeds 10%, devitrification is likely to result. It is preferably at most 5%, more preferably at most 4%.
CaO is essential as a component to lower the viscosity of the molten glass at the time of melting the glass. If it is less than 1%, the viscosity of the molten glass at the time of melting tends to be too high. It is preferably at least 5%. If it exceeds 10%, devitrification is likely to result. It is preferably at most 9%.
SrO is not essential, but may be incorporated up to 9% to lower the viscosity of the molten glass at the time of melting the glass. If it exceeds 9%, KIC/d tends to be small. It is preferably at most 6.5%, more preferably at most 4%.
BaO is not essential, but may be incorporated up to 9% to lower the viscosity of the molten glass at the time of melting the glass. If it exceeds 9%, KIC/d tends to be small. It is preferably at most 4%, more preferably at most 2.5%, particularly preferably at most 2%. It is preferred not to contain BaO, so long as there will be no problem in the viscosity of the molten glass at the time of melting the glass.
If the total content of MgO, CaO, SrO and BaO is less than 10%, the viscosity of the molten glass at the time of melting tends to be too high. Preferably, it is at most 12%. If it exceeds 25%, KIC/d tends to be too small. It is preferably at most 17%, more preferably at most 15%.
ZnO is not essential, but may be incorporated up to 5% to lower the viscosity of the molten glass at the time of melting the glass. If it exceeds 5%, the glass is likely to be reduced during float forming, thus leading to defects of the product.
Na2O and K2O are components to lower the viscosity of the molten glass at the time of melting the glass, and at least one of them must be contained. If the total content of Na2O and K2O is less than 5%, the viscosity of the molten glass at the time of melting tends to be too high. It is preferably at least 7%, more preferably at least 9%. If it exceeds 15%, Na ions and/or K ions are likely to migrate to the electrodes formed on the glass substrate, whereby the electrode characteristics tend to deteriorate. It is preferably at most 13%. The content of Na2O is preferably at most 7%. If it exceeds 7%, the electrical insulating property is likely to deteriorate.
Li2O is not essential, but may be incorporated up to 2% to lower the viscosity of the molten glass at the time of melting the glass. If it exceeds 2%, the strain point is likely to be low.
ZrO2 is not essential, but may be incorporated up to 10% to increase the strain point or to increase KIC/d. If it exceeds 10%, the density tends to be too large, and KIC/d tends to be too small. It is preferably at most 7%, more preferably at most 5%, particularly preferably at most 3%.
A glass substrate according to a preferred embodiment of the present invention consists essentially of the above components. However, other components may be incorporated in a total amount of up to 5% within a range not impair the purpose of the present invention. For example, SO3, F, As2O3, Sb2O3, etc. may be incorporated in a total amount of up to 1% to improve the melting property, refining or forming property of the glass. Further, TiO2 and CeO2 may be incorporated in an amount of 2% each and in a total amount of up to 2%, for example, to prevent browning due to electron rays in PDP. Further, Fe2O3, NiO, CoO, etc. may be incorporated in a total amount of up to 1% to color the glass.
The glass substrate of the present invention can be produced, for example, as follows. Namely, commonly employed starting materials are mixed to have a desired composition, and the mixture is heated and melted in a melting furnace at a temperature of from 1,500 to 1,600xc2x0 C. Bubbling, addition of a refining agent or stirring is carried out to homogenize the glass, and the glass is formed into a predetermined thickness by a conventional float method, then annealed and cut into a predetermined size to obtain a glass substrate.
The glass substrate of the present invention is useful as a substrate for a flat panel display such as PDP or FED.